DEM-Based Stream and Watershed Delineation

Francisco Olivera, Ph.D., P.E.Texas A&M University

Department of Civil Engineering

Watershed Delineation

Watershed delineation is the process of identifying the drainage area of a point or set of points.

For many years, Civil Engineers have been using USGS paper maps for watershed delineation.

The maps are in Universal Transverse Mercator (UTM) coordinates and in 1:24,000 scale.

Watershed Delineation

Water flows in the direction of the terrain steepest downhill slope.

Streamlines are orthogonal to the elevation contour lines.

Watershed Delineation

Streamlines do not flow towards drainage divides, and do not intersect them.

Drainage divides are found along the highest points of the terrain.

DEMs

Digital Elevation Models (DEMs) are grids of elevation.

DEMs store the same type of information contour lines do, but with a different data structure.

Watershed delineation can be based on DEMs rather than contour lines.

30-Meter DEMs

For the United States, 30-meter DEMs have been developed by USGS.

30-meter DEMs are consistent with 1:24,000 paper maps.

30-Meter DEMs

720

700

680

740

680700720740

720 720

30m ~ 90m

30-Meter and 3" DEMs

Hydrologic Functions

Flow direction codes

1

248

16

32 64 128

The eight-direction pour point algorithm (D-8) assigns a flow direction code to each cell, based on the steepest downhill slope as defined by the DEM.

The flow direction code indicates the cell – out of the eight neighbor cells – towards which the water flows.

Flow direction cannot be defined for cells within a terrain depression. DEMs have to be filled before determining flow directions.

67 56 49

53 44 37

58 55 22

1

67 56 49

53 44 37

58 55 22

1

60.152

2244 =−00.7

1

3744 =−Slope:

Hydrologic Functions

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM

22

2 2 2

2

4 4

4 4

1 1 2 4 8

128

128 1 2 4

128 1 41

128

Flow direction grid Flow network(not a grid)

Function: Flow directionArgument: DEM

Flow Direction

Water flows to one of its neighbor cells according to the direction of the steepest descent.

Flow direction takes one out eight possible values.

0 0 0 0 0

0 1 1 2 1

0

0

0 0 0

3 8 5 2

1 1 20 0

1 24

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM Flow network(not a grid)

Function: Flow accumulationArgument: Flow direction

Flow accumulation grid

Flow Accumulation

Flow accumulation is a measure of the drainage area in units of grid cells.

The cell itself is not included.

0 0 0 0 0

0 1 1 2 1

0

0

0 0 0

3 8 5 2

1 1 20 0

1 24

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM Flow accumulation(> 2 cells)

Process: Stream delineationArgument: Flow accumulation

Stream Network

All grid cells draining more than a user-defined threshold value (blue streams) are part of the stream network.

All grid cells located downstream of user-defined cells (red streams) are also part of the stream network.

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM Delineated streams

Function: Stream linksArgument: Delineated streams

Streams links

Stream links are assigned an identification number at random.

Stream links

Stream links are the segments of a stream channel connecting two successive junctions, a junction and an outlet, or a headwater and a junction.

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM Stream links

Process: OutletsArgument: Stream links

Outlets

Outlets are assigned the identification number of their link.

Watershed Outlets

The most downstream cells of the stream segments (yellow/brown cells) are watershed outlets.

User-defined points (red dots) are also watershed outlets.

Modified Stream Links

User-defined outlets modify the stream links by subdividing the segment in which they are located into two segments.

Hydrologic Functions

71

56

445369

74

78 72 69

4768

58 55

21

31

67

58

49 46

37 38

64 22

61 16

DEM Outlets

Function: WatershedArgument: Flow direction and Outlets

Watersheds

Watersheds are assigned the identification number of their outlet.

Watershed Delineation

The drainage area of each watershed outlet is delineated.

Raster to Vector ConversionThe grids of stream links and watersheds are vectorized to obtain stream line and watershed polygon datasets.

Both datasets include a field Grid-Code in their attribute tables, which stores the stream link and watershed identification number.

Since the identification number of the streams and their corresponding watershed is the same, the attribute tables can be related.

Dissolving Spurious Polygons

Cells connected to the main watershed polygon through a corner are defined as a separate polygon (spurious polygon).

These polygons are dissolved into the main polygon.

Merging Watersheds

Adjacent watershed polygons can be merged into a single polygon, if they share their outlet or one flows into the other.

Merging Watersheds

The attribute tables are updated so that the watershed code (WshCode) of the streams is modified and the area of the new polygon is calculated.